Power device package comprising metal tab die attach paddle (DAP) and method of fabricating the package

ABSTRACT

A metal tab die attach paddle (DAP) disposed between the lead frame and a power device die in a power device package reduces the stress exerted on the semiconductor power device die caused by the different coefficients of thermal expansion (CTE) of the semiconductor power device die and the lead frame. In addition the power device package substantially prevents impurities from penetrating into the power device package by increasing the surface creepage distance of a sealant resulting from the metal tab DAP and an optional swaging of the lead frame.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/970,911 filed Jan. 8, 2008, which claims the benefit of Korean PatentApplication No. 10-2007-0002184, filed on Jan. 8, 2007, in the KoreanIntellectual Property Office, the specification of which is herebyincorporated in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power device packages and methods offabricating the power device packages, and more particularly, to powerdevice packages which reduce the stress exerted on a power device dieand a method of fabricating the power device packages.

2. Description of the Related Art

One type of conventional power device package has at least onesemiconductor power device bonded directly to a lead frame and sealedwith an molding resin such as an epoxy molding compound (EMC).

However, when the semiconductor die is directly bonded to the lead frameas in this type of conventional power device package, the semiconductordie is stressed by a temperature change since the coefficient of thermalexpansion (CTE) of a semiconductor die is substantially different fromthat of a lead frame on which the semiconductor die is bonded. As such,the semiconductor die is deformed by the stress, and the performance ofthe semiconductor die usually deteriorates. Moreover, the stress exertedon the semiconductor die results in an inferior and deformed powerdevice package. In addition, the deformed power device packages cancrack and foreign substances such as water can penetrate into the powerdevice package through the cracks.

FIG. 1 is a cross-sectional view illustrating a conventional powerdevice package.

Referring to FIG. 1, a silicon power device die 30 is bonded to a leadframe 10 using a conductive adhesive material 20, and the lead frame 10and the silicon power device die 30 are sealed with a sealant 40 such asan EMC. Conventionally, the lead frame 10 is formed of copper (Cu), andthe CTE of the copper is 17 ppm/° C. However, the CTE of the siliconpower device die 30 is 2-3 ppm/° C.

If the temperature of the conventional power device package increasesdue to heat generated during operation of the conventional power devicepackage or applied to the conventional power device package from anoutside heat source, the power device die 30 is stressed. The lead frame10 expands more than the semiconductor die 30 and, as a result, thesemiconductor die 30 can be deformed (strained).

In the conventional power device package illustrated in FIG. 1, thesealant 40 has a short surface creepage distance from the outside of theconventional power device package to the silicon power device die 30.Therefore, the sealant 40 can easily crack off from the lead frame 10and the silicon power device die 30 due to the difference in the CTEs ofthe silicon power device die 30 and the lead frame 10. Foreignsubstances such as water can penetrate into the silicon power device die30 through the cracks resulting in defective power device packages.

In addition, since the conventional power device package has the siliconpower device die 30 directly bonded to the lead frame 10, the size ofthe silicon power device die 30 is limited to the size of the lead frame10.

SUMMARY OF THE INVENTION

The present invention provides a power device package and a method offabricating the power device package which reduces stress exerted on asemiconductor die, in which the stress is caused by the differencebetween a coefficient of thermal expansion (CTE) of the semiconductordie and that of a lead frame.

According to an aspect of the present invention, there is provided apower device package including a lead frame, a metal tab die attachpaddle (DAP) attached to the lead frame, and a power device die bondedto the metal tab DAP.

The metal tab DAP may be attached to the lead frame using a firstconductive adhesive material, the power device die may be bonded to themetal tab DAP using a second conductive adhesive material, and a meltingpoint of the first conductive adhesive material may be higher than thatof the second conductive adhesive material. The metal tab DAP mayinclude a plated portion in order to improve the wettability of themetal tab DAP for the first and second conductive adhesive materials,and the first and second conductive adhesive materials may be attachedto the plated portion. The first and second conductive adhesivematerials may be formed of a solder wire or solder paste.

The metal tab DAP may have a CTE lower than that of the lead frame. Themetal tab DAP may have a CTE to compensate for a difference between aCTE of the lead frame and a CTE of the power device die so as tominimize a stress exerted on the power device die. The metal tab DAP mayhave an area larger than that of the lead frame so as to allow a largerpower device die to be bonded to the metal tab DAP.

The package may further include a sealant surrounding the lead frame,the metal tab DAP, and the power device die so as to seal the powerdevice die, the sealant having a long surface creepage distance from anoutside of the package to the power device die due to the metal tab DAP.

The lead frame may be formed of Cu, and the metal tab DAP may be formedof an alloy of Fe and Ni having a CTE lower than that of Cu, wherein themetal tab DAP may be attached to the lead frame using a first conductiveadhesive material, the power device die may be bonded to the metal tabDAP using a second conductive adhesive material, and a melting point ofthe first conductive adhesive material is higher than that of the secondconductive adhesive material.

The metal tab DAP may include a Cu-plated portion and a Ag-platedportion in order to improve the wettability of the metal tab DAP for thefirst and second conductive adhesive materials, the first conductiveadhesive material may be attached to the Cu plated portion, and thesecond conductive adhesive material may be attached to the Ag platedportion. The metal tab DAP may have a CTE lower than that of the leadframe. The metal tab DAP may have an area larger than that of the leadframe.

The lead frame may be a single gauged lead frame having the samethickness as an outside lead or a dual gauged lead frame having adifferent thickness than the outside lead. The power device package maybe a surface mount device type package.

According to another aspect of the present invention, there is provideda method of fabricating a power device package, the method includingforming a metal tab DAP on a lead frame, forming a power device die onthe metal tab DAP, and surrounding the lead frame, the metal tab DAP,and the power device die with a sealant so as to seal the power devicedie.

The metal tab DAP may be attached to the lead frame using a firstconductive adhesive material, and the power device die may be bonded tothe metal tab DAP using a second conductive adhesive material, a meltingpoint of the first conductive adhesive material is higher than that ofthe second conductive adhesive material.

The metal tab DAP may include a plated portion so as to improve thewettability of the metal tab DAP for the first and second conductiveadhesive materials, and the first and second conductive adhesivematerials may be attached to the plated portion. The metal tab DAP mayhave a CTE to compensate for a difference between a CTE of the leadframe and a CTE of the power device die so as to minimize a stressexerted on the power device die.

The metal tab DAP may have an area larger than that of the lead frame soas to allow a larger power device die to be formed on the metal tab DAP.The sealant may include a long surface creepage distance from an outsideof the package to the power device die due to the metal tab DAP.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a cross-sectional view of a conventional power device package;

FIG. 2A is a plan view of a power device package including a metal tabdie attach paddle (DAP), according to an embodiment of the presentinvention;

FIG. 2B is a cross-sectional view taken along line □-□′ of FIG. 2A;

FIG. 3 is an exploded sectional view of select elements illustrated inFIG. 2B;

FIGS. 4A, 5A, and 6A are plan views illustrating a method of fabricatinga power device package according to an embodiment of the presentinvention; and

FIGS. 4B, 5B, and 6B are sectional views taken along lines II-II′,III-III′, and IV-IV′ of FIGS. 4A, 5A, and 6A, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully with reference tothe accompanying drawings according to embodiments of the presentinvention. In the drawings, the thicknesses of layers and regions are insome cases distorted for clarity.

FIG. 2A is a plan view of a power device package including a metal tabdie attach paddle (DAP) 200, according to an embodiment of the presentinvention.

Referring to FIG. 2A, the power device package includes a lead frame 100(shown in FIG. 2B), a metal tab die attach paddle (DAP) 200 attached tothe lead frame 100, a power device die 300 bonded to the metal tab DAP200, and a sealant 400 (shown in outline) sealing the metal tab DAP 200and the power device die 300, to which an outside lead 600 iselectrically connected to the power device die 300 through bonding wires220. A tie bar 500 formed at an upper portion of the power devicepackage is used in the package forming process and is removed at the endof the process.

The power device package according to an embodiment of the presentinvention includes the power device die 300 bonded to the metal tab DAP200, hence, the power device die 300 is not directly bonded to the leadframe 100. The metal tab DAP 200 is attached to the lead frame 100 toreduce stress exerted on the power device die 300 due to stress causedby the difference between the coefficient of thermal expansion (CTE) ofthe power device die 300 and the lead frame 100. The power devicepackage according to an embodiment of the present invention will now bedescribed more fully with reference to FIG. 2B.

Referring to FIG. 2B, the metal tab DAP 200 is attached to the leadframe 100, the power device die 300 is bonded to the metal tab DAP 200,and a sealant 400 seals the power device die 300.

The metal tab DAP 200 is attached to the lead frame 100 using a firstconductive adhesive material 150, and the power device die 300 is diebonded to the metal tab DAP 200 using a second conductive adhesivematerial 250. The metal tab DAP 200 may be formed of a material whichreduces the stress exerted on the power device die 300. For example, incase of a conventional lead frame formed of copper, the metal tab DAP200 may be formed of an alloy of iron (Fe) and nickel (Ni). The CTE ofthe alloy is between that of the lead frame 100 and that of the powerdevice die 300. Hence, the metal tab DAP 200 may have a CTE that reducesthe stress caused by the difference in thermal expansion between thelead frame 100 and the power device die 300. The thickness of the metaltab DAP 200 may appropriately be formed by considering stress reductionand the thickness of the whole power device package. For example, thethickness of the metal tab DAP 200 may be 0.20-0.50 μm.

During the packaging process, the metal tab DAP 200 is attached to thelead frame 100, and then, the power device die 300 is die bonded to themetal tab DAP 200. Therefore, the melting point of the first conductiveadhesive material 150 may be higher than that of the second conductiveadhesive material 250. The first conductive adhesive material 150attaches the metal tab DAP 200 to the lead frame 100. The secondconductive adhesive material 250 bonds the power device die 300 to themetal tab DAP 200. The first and second conductive adhesive materials150 and 250 may be solder wire or solder paste. The first and secondconductive adhesive materials 150 and 250 may be formed of a solderwhich is an alloy of Pb, Sn, and Ag. The first and second conductiveadhesive materials 150 and 250 may have an appropriate thickness toinsure a firm bond within the constraints of the thickness of the wholepackage. The thickness of the first and second conductive adhesivematerials 150 and 250 may be about 0.5-3 mm.

The metal tab DAP 200 should have an efficient wettability for the firstand second conductive adhesive materials 150 and 250. The efficientwettability makes it possible for the metal tab DAP 200 to be firmlyattached to the lead frame 100 and for the power device die 300 to befirmly bonded to the metal tab DAP 200. In order to improve thewettability of the metal tab DAP 200, a portion of the metal tab DAP 200may be plated such that the portion contacts the first and secondconductive adhesive materials 150 and 250. For example, a portion of themetal tab DAP 200 is plated with Cu, which contacts the first conductiveadhesive material 150, and a portion is plated with Ag, which contactsthe second conductive adhesive material 250.

For a conventional power device package, the bonding force of thesealant 400 and surface creepage distance of the sealant 400 areincreased by a swaging C to the lead frame 100, so that impurities aresubstantially prevented from penetrating into the power device die 300.However, increasing the surface creepage distance of the sealant 400 islimited since the power device die 300 is directly bonded to the leadframe 100.

The power device package according to an embodiment of the presentinvention includes the metal tab DAP 200 attached to the lead frame 100and the power device die 300 bonded to the metal tab DAP 200, so thatthe bonding force of the sealant 400 is increased by an extended bondingarea of the sealant 400 resulting from the DAP 200 and the swaging C. Inaddition, impurities are more effectively prevented from penetratinginto the power device die 300 through a surface creepage B since thesurface creepage B distance of the sealant 400 is increased from theoutside of the package to the power device die 300 as compared with thatof the conventional power device package illustrated in FIG. 1.

FIG. 3 is an exploded sectional view illustrating the relative widths ofthe power device die 300, the metal tab DAP 200, and the lead frame 100of the power device package illustrated in FIG. 2B.

Referring to FIG. 3, the relative widths W_(L), W_(T), and W_(D) of thelead frame 100, the metal tab DAP 200, and the power device die 300 areillustrated, respectively. For a conventional power device package, thesize of the power device die 300 is limited according to the size of thelead frame 100 bonded to the power device die 300.

However, the power device package according to an embodiment of thepresent invention includes the metal tab DAP 200 attached to the leadframe 100, and the power device die 300 bonded to the metal tab DAP 200.Therefore, the size of the power device die 300 can be accommodated bycontrolling the size of the metal tab DAP 200 attached to the lead frame100. Hence, the width of the power device die 300 can be greater thanthat of the lead frame 100. Even though only the widths are illustratedin FIG. 3, the lengths of the power device die 300, the metal tab DAP200, and the lead frame 100 have the same relative sizes as the widthsillustrated in FIG. 3. Therefore, the whole area of the surface of themetal tab DAP 200 bonded to the power device 300 can be controlled byselecting the width W_(T) of the metal tab DAP 200 and the length of themetal tab DAP 200 so that the whole area is optimized for each type ofpower device die 300.

An effect of adding the metal tab DAP 200 will now be described withreference to the accompanying simulation data, in which the effect ofadding the metal tab DAP 200 is simulated by adding the metal tab DAP200 to a power device package according to an embodiment of the presentinvention.

Table 1 shows the stress simulation data. The size of the chip used inthe simulation is 3580×2800×300 μm³, and the size of the metal tab DAPis 5.4×4.0×0.5 mm³. The thicknesses of first and second conductiveadhesives are 50 μm and 30 μm, respectively, and are formed of an alloyof Pb, Sn, and Ag. For the simulation, it is assumed that the powerdevice package has a uniform temperature dispersion, a non-separation ofan epoxy molding compound (EMC) of the sealant and a lead frame, and anon-separation of the EMC of the sealant and the chip. In addition it isassumed that there are no voids in the solder, no water absorption, auniform thickness of each bonding layer, and normal sawingcharacteristics.

TABLE 1 Current Model Package Adding part - Copper Adding part - AlloySolder Solder Solder Chip Mises Plastic Chip Mises Plastic Chip MisesPlastic Principal Stress Strain Principal Stress Strain Principal StressStrain stress (MPa) (%) stress (MPa) (%) stress (MPa) (%) 175° C.−>25°C. 26.4 47.4 1.0 27.3 47.8 1.03 6.4 40.0 0.4  25° C.−>260° C. 225.5105.4 5.64 185.0 60.3 2.03

In Table 1, the “Current Model Package” column is a conventional powerdevice package with a chip directly bonded to a conventional lead frame,the “Adding part—Copper” column is a power device package including ametal tab DAP formed of Cu, and the “Adding part—Alloy” column is apower device package including a metal tab DAP formed of an alloy of Feand Ni. The temperature conditions include a temperature variation froma molding temperature of 175° C. to a room temperature of 25° C. andanother temperature variation from a room temperature of 25° C. to ahigh operating temperature of 260° C.

Table 1 illustrates that the stress exerted on a die of the metal tabDAP formed of alloy is remarkably lower than that of the current modelpackage. The solder is an adhesive material applied on a portion of themetal tab DAP on which the die is formed. The stress and the strainexerted on the solder of the metal tab DAP formed of alloy are alsoremarkably lower than those of the current model package. However, themetal tab DAP formed of Cu is not significantly different from thecurrent model package of a power device package since the current modelpackage also includes a copper layer on which the die is formed.

The stress simulation data illustrates that stress exerted on the chipand the solder can be efficiently reduced by applying an appropriatemetal tab DAP to the power device package.

Table 2 illustrates the thermal resistance simulation data.

TABLE 2 Rthja (° C./W) Normal KFC ½H lead +PMC90 Tab +Alloy Tab frame0.25 mm 0.38 mm 0.25 mm 0.38 mm minimum 107.2 105.8 105.2 106.9 106.7land pattern 1 in² Cu 38.5 38.3 38.3 39.6 40.0 plane

In Table 2, the “Normal KFC 1/2H lead frame” column is without a metaltab DAP, the “PMC90 Tab” column is with a metal tab DAP formed of Cu,and the “Alloy Tab” column is with a metal tab DAP formed of alloy of Feand Ni. In addition, the metal tab DAPs have thicknesses of 0.25 mm and0.38 mm. Rthja is an abbreviation for Thermal Resistance betweenJunction to Ambient, in units of ° C./W. The “minimum land pattern” rowis without a heat emitting thermal plane, and the “1 in² Cu Plane” rowis with a Cu thermal plane. The thermal conductivity of the normal KFC1/2H lead frame by itself is 364.5 W/m ° C., and the thermalconductivity of the alloy tab by itself is 25.8 W/m ° C.

Referring to Table 2, even if the thermal conductivity of the alloy tabis lower than that of the normal KFC 1/2H lead frame, the Rthja of thepower device package including the alloy tab is not significantlydifferent from those of the PMC 90 tab and the normal KFC 1/2H leadframe. Thus, the alloy tab can be applied to the power device packagewithout affecting the Rthja of the power device package.

FIGS. 4A, 5A, and 6A are plan views illustrating a method of fabricatinga power device package, according to an embodiment of the presentinvention, and FIGS. 4B, 5B, and 6B are sectional views taken alonglines II-II′, III-III′, and IV-IV′ of FIGS. 4A, 5A, and 6A,respectively.

Referring to FIGS. 4A and 4B, the lead frame 100 is prepared, and formedby a swaging process. As described above, the tie bar 500 and theoutside lead 600 are connected to the lead frame 100. The phantom-linedbox represents a region sealed by the sealant 400.

Referring to FIGS. 5A and 5B, the first conductive adhesive material 150is disposed on the lead frame 100, and then the metal tab DAP 200 isdisposed on the first conductive adhesive material 150. As describedabove, the first conductive adhesive material 150 may be a solder paste.The bottom of the metal tab DAP 200 contacting the first conductiveadhesive material 150 may be plated in order to improve the wettabilityof the metal tab DAP 200.

Referring to FIGS. 6A and 6B, the second conductive adhesive material250 is disposed on the metal tab DAP 200, and then the power device die300 is disposed on the second conductive adhesive material 250. Asdescribed above, the melting point of the second conductive adhesivematerial 250 may be lower than that of the first conductive adhesivematerial 150. The top surface of the metal tab DAP 200 contacting thesecond conductive adhesive material 250 may be also plated in order toimprove the wettability of the metal tab DAP 200.

After the power device die 300 is die bonded, the power device die 300is connected to the outside lead 600 through the bonding wires 220, andthen, the sealant 400 surrounding the lead frame 100, the metal tab DAP200, and the power device die 300 is formed to complete the power devicepackage prior to the removal of the tie bar 500.

As described above, the power device package according to an embodimentof the present invention has various advantages since the metal tab DAPis placed between the power device die and the lead frame. With themetal tab DAP the stress exerted on the power device die is reduced, thesurface creepage distance of the sealant is increased, and the size ofthe semiconductor chip in the power device package can be increased.

The power device package according to the present inventionsignificantly reduces the stress exerted on the semiconductor powerdevice die by adding the metal tab DAP between the power device die andthe lead frame, in which the stress is caused by the difference betweenthe CTE of the semiconductor power device die and the lead frame.

In addition, the power device package according to the present inventioninhibits impurities from penetrating into the power device die since thesurface creepage distance of the sealant is increased by adding themetal tab DAP.

Furthermore, the power device package according to the present inventioncan accommodate various sized power device dies by adjusting the size ofthe metal tab DAP regardless of the size of the lead frame since thepower device die is bonded to the metal tab DAP.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby one of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

The invention claimed is:
 1. A method of fabricating a power devicepackage, the method comprising: attaching a metal tab die attach paddleto a lead frame, wherein the lead frame comprises a protruding swagingportion and the die attach paddle has a low coefficient of thermalexpansion; die bonding a power device die to the metal tab die attachpaddle; and surrounding the lead frame, the metal tab die attach paddle,and the power device die with a sealant so as to seal the power devicedie.
 2. The method of claim 1, wherein the metal tab die attach paddlehas an optimal coefficient of thermal expansion (CTE) to compensate fora difference between a CTE of the lead frame and a CTE of the powerdevice die so as to minimize a stress exerted on the power device die.3. The method of claim 1, wherein the metal tab die attach paddle has anarea larger than that of the lead frame so as to allow a larger powerdevice die to be bonded to the metal tab die attach paddle.
 4. Themethod of claim 1, wherein the sealant comprises an increase in thesurface creepage distance from an outside of the package to the powerdevice die due to the metal tab die attach paddle.
 5. A method offabricating a power device package, the method comprising: attaching ametal tab die attach paddle to a lead frame; die bonding a power devicedie to the metal tab die attach paddle; and surrounding the lead frame,the metal tab die attach paddle, and the power device die with a sealantso as to seal the power device die; wherein the metal tab die attachpaddle is attached to the lead frame using a first conductive adhesivematerial, the power device die is bonded to the metal tab die attachpaddle using a second conductive adhesive material, and a melting pointof the first conductive adhesive material is higher than that of thesecond conductive adhesive material.
 6. The method of claim 5, whereinthe metal tab die attach paddle comprises a plated portion so as toimprove wettability of the metal tab die attach paddle for the first andsecond conductive adhesive materials, and the first and secondconductive adhesive materials are attached to the plated portion.
 7. Themethod of claim 5 wherein the leadframe comprises a protruding swagingportion.
 8. A method of fabricating a power device package, the methodcomprising: attaching a metal tab die attach paddle to a lead frame; diebonding a power device die to the metal tab die attach paddle; andsurrounding the lead frame, the metal tab die attach paddle, and thepower device die with a sealant so as to seal the power device die;wherein the lead frame is formed of Cu, and the metal tab die attachpaddle is formed of an alloy of Fe and Ni having a coefficient ofthermal expansion (CTE) lower than that of Cu, wherein the metal tab dieattach paddle is attached to the lead frame using a first conductiveadhesive material, the power device die is bonded to the metal tab dieattach paddle using a second conductive adhesive material, and a meltingpoint of the first conductive adhesive material is higher than that ofthe second conductive adhesive material.
 9. A method of fabricating apower device package, the method comprising: attaching a metal tab dieattach paddle to a lead frame; die bonding a power device die to themetal tab die attach paddle; and surrounding the lead frame, the metaltab die attach paddle, and the power device die with a sealant so as toseal the power device die; wherein the metal tab die attach paddle has aCTE lower than that of the lead frame and is formed of an alloy of Feand Ni.